The present invention pertains to linear beam or "O" type cavity microwave tubes.
The term "linear-beam microwave tubes" refers to a tube that uses a focusing magnetic field that is substantially parallel to the path of the electrons of the beam. These tubes make use of the interaction between the electrons of the beam that are moving together and a microwave.
These tubes may be klystrons or coupled-cavity travelling-wave tubes and their derivatives.
A standard klystron has an electron gun that produces a long and thin electron beam through a sequence of cavities connected to one another by drift tubes. At the output of the last cavity, the electrons are gathered in a collector that is coaxial with the beam. This collector gets heated and it is cooled, for example by making a cooling fluid circulate at its periphery.
A focusing device surrounds the cavities. It prevents the electrons from diverging. This focusing device is often formed by an electromagnet in the form of a hollow cylinder.
A microwave signal to be amplified is introduced into the cavity nearest to the gun. The output cavity or the cavity closest to the collector is designed to be connected to a user device by means of a transmission line, this transmission line conveying the amplified microwave signal towards the user device. This transmission line is a rectangular, circular or coaxial waveguide.
This waveguide is generally positioned transversally to the electron beam. The coupling between the output of the cavity and the waveguide is done by at least one hole in the side wall of the cavity.
A window may block the coupling hole. It is designed to let through the microwave signal extracted while at the same time maintaining the high vacuum that prevails within the cavity.
Since the transmission line is connected to a side wall of the output cavity, the focusing device must take account of this link and must include a notch at this place. The magnetic field is reduced and dissymmetrical at the output cavity while this is the place where it is most needed. Consequently, the electron beam is defocused.
This transversal transmission line also gives rise to a considerable difficulty during the installation of the tube. The assembly formed by the gun, the cavities and the collector has to be slid into the focusing device and the relative position of the assembly and of the device has to be adjusted in order to fix the transmission line. This operation is very delicate because of the weight involved and the fragile nature of the link. The assembly formed by the gun, cavities and collector weighs several hundreds of kilograms.
Proposals have already been made to overcome these drawbacks in the magnetic field and simplify the assembly by using a transmission line that surrounds the collector. However, this arrangement has a major drawback. The collector is limited in size and hardly accessible. It is difficult to cool and therefore costly. This configuration is reserved for low-power tubes.
The present invention seeks to make a linear-beam cavity microwave tube that has neither a dissymmetry of the magnetic field nor a small-sized collector, is very simple to mount and costs little.
To achieve these ends, the present invention proposes to make the microwave signal to be extracted and the electrons of the beam exist together in the collector.